This invention relates generally to power systems, and more particularly to power systems for providing an assured, or uninterruptible, supply of electrical power to one or more critical loads. More particularly still, the invention relates to such power systems employing fuel cells as a source of electrical power.
By far, the most common source of electrical power for a great variety of loads is via the extensive power grid provided by the various electric utilities. The electrical power available on the utility grid is generally fairly reliable as to continuity and adherence to established standards of voltage, frequency, phase, etc. However, from time to time discontinuities and/or departure from those standards do occur. If they are brief or modest, most loads are relatively insensitive to those events. On the other hand, there are a growing number of loads which are relatively intolerant of even brief aberrations in the power supplied by the utility grid, with the principal example being computers and various types of electronic data processing devices. Even brief interruptions in the standardized supply of electric power by the utility grid may cause the computer to malfunction, with sometimes costly, and always bothersome, consequences.
In defining this concern, the Computer Business Equipment Manufacturers Association (formerly CBEMA, and now ITI) has developed a set of Power Acceptability Curves which establish the standards, or at least provide guidance, for determining the power norms which will assure continued operation of those types of loads. In that regard, a standard has been adopted indicating that a computer can tolerate a one half cycle or 8.3 ms power interruption. The power available on the utility grids is not presently capable of meeting this standard on a substantially continuous basis. Accordingly, it has been and is, necessary to provide supplemental power sources if it is important to assure that critical loads have a substantially continuous or uninterrupted supply of electrical power. For purposes of this application, a supply of power with interruptions or transfers of no greater than 8.3 ms duration, may be referred to as being xe2x80x9cseamlessxe2x80x9d, xe2x80x9csubstantially continuousxe2x80x9d, or xe2x80x9csubstantially uninterruptedxe2x80x9d.
Referring to FIG. 1, there is illustrated one existing form of uninterruptible power supply (UPS), a so-called xe2x80x9con-linexe2x80x9d or xe2x80x9cdouble conversionxe2x80x9d type, used to supply a critical load in those instances when the utility grid supply is interrupted or is outside of specified limits. The utility grid power supply normally appears on conductor 110, and is passed via normally-closed contacts of a 3-pole transfer switch 112 to a rectifier 120, which supplies the critical loads 114 via an inverter 122. However, to provide continued power in and during those intervals when the utility grid power is not within the specified limits, a backup battery 116 is provided to supply immediate power of limited duration, and an emergency electrical generator 118 is then connected to the other contact of transfer switch 112 to follow-up with a longer term temporary supply. To accommodate the use of battery 116 in a system which relies on AC power for the loads 114, it is necessary to provide the rectifier 120 to charge battery 116 and the inverter 122 to convert the DC supply from the battery to the necessary AC supply for the loads. A high speed switch 124 connected between the transfer switch 112 and the loads 114 operates as a bypass switch to provide temporary power if the inverter 122 or rectifier 120 must be serviced. Because the grid and loads are not normally directly connected, but rather the power to the loads is required to pass through a pair of converters with the aid of the UPS battery, this type of UPS is termed an xe2x80x9cin-linexe2x80x9d or xe2x80x9cdouble conversionxe2x80x9d type. This arrangement, though effective, requires a number of costly components that are in use only during the intervals when the utility grid power is unsatisfactory.
Another arrangement of a power system for providing substantially uninterrupted power to critical loads is described in PCT application US99/10833 for xe2x80x9cPower Systemxe2x80x9d, published on Nov. 25, 1999 as WO 99/60687. Referring to FIG. 2 in the present application, the relevant portions of the invention described in that PCT application are depicted in a very simplified, generalized form, with elements being numbered such that their last 2 digits are the same as their functionally equivalent counterparts in FIG. 1. The critical loads 214 receive substantially uninterrupted power from a motor-generator 230 within an uninterruptable power system module 231, which module also contains transfer switches, rectifiers and inverters. Several alternative electrical power sources are provided to maximize the continued powering of the motor-generator 230. One such power source may be the utility grid 210. Another source may be the fuel cell generator power plant 218. A transfer switching arrangement 212 enables one or the other of the utility grid 210 and the fuel cell 218 to normally provide the power to drive the motor-generator 230. This type of uninterruptible power supply is also of the xe2x80x9con-linexe2x80x9d or xe2x80x9cdouble conversionxe2x80x9d type inasmuch as the grid is not directly connected to the loads 214, but acts through the rectifier and inverter converters and the flywheel and/or fuel cells to energize motor-generator 230 which in turn provides uninterrupted power. In fact, the fuel cell 218 is configured to operate in a grid connect (G/C) mode with the utility grid 210 for system economy, so in grid connected mode both the grid and the fuel cell supply the xe2x80x9cgridxe2x80x9d terminals of the transfer switch. In the event of failure of the grid supply 210, the fuel cell 218 is intended to serve as the continuing power source for the motor-generator 230. However, in such event, the fuel cell 218 must reconfigure from a xe2x80x9cgrid connectxe2x80x9d (G/C) mode of operation to a xe2x80x9cgrid independentxe2x80x9d (G/I) mode. The power conditioning system (PCS) portion of the fuel cell 218 includes associated inverters, switching transistors and breakers (not shown) that effect the conversion of DC power to AC power and that govern the fundamental G/C and G/I modes of fuel cell operation. That mode transition (from G/C to G/I) has typically required the fuel cell 218 and transfer switch 212 to interrupt power generation for up to 5 seconds. Such interruption is not xe2x80x9cseamlessxe2x80x9d, and would be of unacceptable duration for critical computer loads 214. Accordingly, a backup flywheel power source 216 provides immediate power of limited duration(similar to the battery source 116 in FIG. 1) to the motor-generator 230 at least during such mode conversions. That backup power source 216 is a flywheel 236 driving a bi-directional AC/DC converter 238. The converter 238 keeps the flywheel spinning during normal operation, and discharges the flywheel 236 during backup operation. The various transfer switches used in the transfer switching arrangement 212 and in the uninterruptable power system module 231 may be electro-mechanical, static, or a combination thereof, and serve to effect the various power switching functions.
While the Power System of the abovementioned PCT application may provide a substantially uninterrupted source of power to various critical loads and may advantageously employ fuel cells as one of the main sources of the power, it nevertheless requires the use of considerable additional equipment that is complex and costly. For example, the separate motor-generator 230, and the backup power source 216 which includes the flywheel 236/converter 238 combination, represent necessary, but expensive, components in order to assure the degree of power continuity sought and required.
Another type of UPS is of the xe2x80x9cstand-byxe2x80x9d type wherein the grid is directly connected to the loads and a stand-by UPS remains idle, even if connected to the loads, until a switch disconnects the grid from the loads. An example of such a system is disclosed in U.S. Pat. No. 6,011,324. The fuel cell and associated inverters are normally connected to the loads, but in an idle standby mode while the grid supplies power directly to the loads. When the grid fails, the fuel cell is rapidly brought to full output power and a solid state switch disconnects the grid. Here, too, a number of costly components are in use only during the intervals when the utility grid power is unsatisfactory. Accordingly, it is a principal object of the present invention to provide a power system for providing a substantially uninterrupted (seamless) supply of electric power to critical loads in a relatively economical manner.
It is a further object to provide such a power system in which one or more fuel cell power plant(s) are utilized to substantially continuously supply power to the loads.
According to the invention, there is provided a relatively economical and reliable power system for providing substantially uninterrupted electric power to one or more critical loads. A first power source, such as the utility grid, provides sufficient power to supply the critical loads. A second power source comprising at least one, and typically multiple, fuel cell power plants, provides sufficient power to supply at least the critical loads. The fuel cell power plant(s) is/are adapted to be, and is/are, substantially continuously connected to the critical loads and are substantially continuously providing significant power to at least the critical loads. A static switch operates to rapidly and seamlessly connect and disconnect the utility grid to the critical load(s) and to the fuel cell power plant(s), for economical continuous usage of the fuel cell power plant(s). Significant economy is realized by having the substantially continuously operating fuel cell(s) substantially continuously connected to the load, and normally also to the grid. In this way, the fuel cells may continuously deliver their rated power, with the requisite portion going to the critical loads and any excess being delivered to non-critical loads and/or the grid. The static switch may be one or more silicon controlled rectifiers (SCRs), or thyristors. Solid-state switch controls operate to rapidly switch the static switch in 4 ms, or less, to make seamless transfers between the first and second power sources. This switching speed is significantly faster than is obtained with conventional line commutation of thyristors. Further control electronics provide high-speed transitions (less than about 4 ms) in the operating modes of the power conditioning system (PCS) inverters associated with each of the fuel cell power plants. This assures that the fuel cell mode transitions, heretofore normally slow, are at a speed comparable to that of the static switch so as to provide substantially seamless power transfers of and between the first and second power sources. This allows continuous productive operation of the fuel cell power plants.
The foregoing features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof as illustrated in the accompanying drawings.